Multi-stroke engine operation (e.g., varying the number of strokes in a combustion cycle) is one method that may improve fuel economy while maintaining reserve torque capacity for elevated load conditions. During Multi-stroke operation, selected strokes of each cylinder may not follow a typical four-stroke cycle for some conditions but rather perform extra compression stokes, for example. Operating an engine in this fashion can extend the torque interval, thereby lowering the average torque available from an engine. However, it can also increase fuel economy, at part load, by increasing the cylinder air amount and thermal efficiency. Further, the cylinder may resume four-stroke operation by simply eliminating any benign pumping strokes after a combustion event in the multi-stroke cylinder. Consequently, the ability to control the stroke count, e.g., 2-stroke, 4-stroke, 6-stroke, and/or 12-stroke, can enable cylinders on-demand to be used to improve fuel economy while retaining torque capacity.
One method to control combustion in cylinders during engine operation is described in Unexamined Japanese Pat. No. S55-29002. This method provides a way of controlling fuel injected into a cylinder that may reduce engine emissions when an engine is operated in a partial cylinder mode. The method controls injected fuel based on a catalyst temperature or oxygen sensor temperature.
However, the inventors herein have recognized that the before-mentioned approach can have several disadvantages. Namely, the approach permits inducted air to cool a catalyst. By allowing air to pass through an engine, stored energy, in the form of heat, is transferred from a catalyst to the atmosphere, resulting in catalyst energy loss. Catalyst efficiency may be reduced if the temperature of a catalyst falls below a certain temperature.
Also, the method may increase vehicle emissions if cylinder deactivation occurs during a condition where a catalyst has a large amount of stored oxidants. For example, if an engine enters fuel shut-off for a short period of time, the amount of oxygen stored in the catalyst may increase. In addition, the catalyst may remain at a temperature high enough to permit cylinder deactivation by the method. If this occurs, higher levels of engine feed gas NOx, produced by cylinder deactivation, may not be efficiently converted because NOx and oxygen can compete for reaction sites in a catalyst.